U.S. Pat. No. 5,051,039 (Heiliger), issued Sep. 24, 1991, relates to a hydraulic steel mine prop that constitutes a piston cylinder device of the type described above. The known mine prop comprises components that can be assembled cold without welding. Hence, high strength steel can be used for making these mine props. The absence of welding heat avoids adversely influencing the texture of the high strength steel. The known mine prop leaves room for improvement with regard to the protection of the outer and inner cylindrical surfaces and with regard to the reduction of friction between these two surfaces that slide relative to each other.
French Patent Publication 1,202,536 (Noe), published on Jan. 11, 1960, discloses a mine prop in which the inner and outer cylindrical surfaces that face each other are provided with a coating of synthetic material which may be of the thermoplastic or thermosetting type. The coating may have a layer thickness within the range of 1 to 4 mm. The coating in this French Publication may be applied, for example, by spraying, and requires a subsequent calibration. Thermoplastic coatings may, for example, be calibrated by a heated piston. The relatively thick coating of about 1 to 4 mm thickness may even be applied to an unworked surface to smooth out the surface roughness.
The use of thermoplastic synthetic materials for providing a protective coating on surfaces of a mine prop as taught by French Patent Publication 1,202,536 (Noe) has substantial disadvantages, because such materials are relatively permeable to oxygen and water vapor, whereby these media which are used as the pressure medium for operating the mine prop, tend to diffuse through the protective coating of a thermoplastic synthetic material, thereby reaching the interface between the thermoplastic material and the metal of which the mine prop is made. As a result, pockets containing water are formed at the interface. These pockets lead to corrosion and in time to the separation of the coating from the metal. As a result, it becomes impossible to operate such a mine prop under the pressures required for its operation. Another drawback of such thick synthetic material coating layers is seen in that the piston motion drives the above mentioned water pockets at the interface between the coating and the metal along the interface in the piston motion direction, thereby speeding-up the peeling of the coating. Further, it is even possible that such water pockets permit the piston to pass the pockets, so that pockets are formed behind the piston in the low pressure chamber. If this happens, the water pocket, which is in effect a water bubble, can explode in the low pressure chamber, thereby damaging the synthetic material coating. Another drawback of protective coatings of synthetic materials that are at least 1 mm thick, is the fact that the coatings are too elastic in the presence of high pressure. This applies to thermoplastic materials as well as to thermosetting materials. As a result, the pressure in the piston chamber compresses the coating in a radial direction to an extent that cannot be accommodated by the sealing elements between the piston and the piston enclosing wall surface, whereby maintaining the required high pressure, for example in a mine prop, becomes difficult if not impossible.
It appears that the French Patent Publication 1,202,536 wants to avoid using heat for applying the coating, whereby especially in connection with thermosetting materials, a cross-linking and evaporation of the solvent would take several months. The use of a solvent is apparently necessary for spraying the thermosetting material onto the relevant surfaces. On the other hand, if it is intended to cross-link the thermosetting material by the application of heat, substantial volumes of the solvent gases would have to be removed in an environmentally satisfactory manner. Another drawback is seen in the fact that it is difficult to apply coatings by spraying to a thickness of one to four mm thickness. In any event, the required calibration of these relatively thick coatings is an undesirable step.
In the French Patent Publication 1,202,536, even where the protection is intended to be accomplished by the insertion of a ready-made sleeve of the synthetic material, it is difficult to avoid sealing problems due to the elasticity of the synthetic materials having a thickness within the range of 1 to 4 mm. Furthermore, in mine props the pressure is applied to only one side of the piston and the opposite chamber is under low pressure. As a result, the formation of a radial step or bulge in the coating is unavoidable by the pressure application, due to the elasticity of the materials. Such a radial step in the coating is highly undesirable, because it would tend to damage the sealing rings between the piston and the cylinder wall.
In connection with mine props it was necessary to provide protection against corrosion by electroplating, especially where the mine props were intended to be operated by water as the pressure medium. As a result it was not possible to use high strength steels for making the components of the mine props, due to the hydrogen embrittlement of such high strength steel alloys. Thus, it has been conventional practice to make the mine props of lesser strength steel with the result that the mine props became even heavier, because the lesser strength steels required larger wall thicknesses. This problem has not entirely been solved by the conventional synthetic material coatings as taught in the above mentioned French Patent Publication 1,202,536, due to the drawbacks described above.
Another disadvantage of electroplating the components of mine props is seen in that the head and foot sections required welding which destroyed the electroplating protection coating. Where mine props with electroplated components have been used anyway, some protection against corrosion has been gained by adding an emulsion to the water used as the pressure medium. However, emulsions have their own drawbacks, especially environmental drawbacks.
Mine props constructed as described in the above mentioned U.S. Pat. No. 5,051,039 (Heiliger) avoid welding connections by using cold connections, thus making it possible to use high strength steels, whereby welding heat no longer adversely affects the strength of the steel. An additional advantage of the Heiliger mine prop is seen in that the use of high strength steels avoids the need for a material removing machining operation of the cylindrical surfaces that need to be protected. Merely a degreasing and/or descaling operation may be necessary.
British Patent 893,050 (Von Linsingen), published Apr. 4, 1962, discloses improvements in hydraulic pit or mine props wherein the inner surface of the cylinder is coated with a coating (16) that is resiliently deformable, yet resistant to corrosion, swelling, and aging, as well as wear and tear. The inner coating is, for example, ethoxylin resin or polyamide, which is a thermoplastic, in a sprayable solution which may contain a slip promoting agent, e.g. molybdenum disulphide. The sprayed on coating has a substantial thickness of about 0.1 mm to 5 mm or 25% to 50% of the wall thickness of the outer cylinder wall, and is thus capable of bulging in response to piston movements in the cylinder. German Patent Publication (DE-OS) 1,812,616 (Schulte et al.), published Jun. 18, 1970, discloses a hydraulic metal cylinder especially for mine props, wherein the inner surface of the cylinder is coated with a synthetic layer of synthetic material having a thickness of about 0.2 mm to about 0.6 mm. The reference teaches that the inner surface of the cylinder is spray-coated, whereby any possible roughness of the inner cylinder surface is coated over by the spray coat. This reference does not teach the kind of synthetic material nor does it address the problem of bulging.
German Utility Model Publication 1,769,418, (Remscheidt), registered Jul. 3, 1958, discloses a cylinder pipe for mine props with several coating layers on the inner and outer surfaces of the pipe. The layers are thickest on the pipe surfaces and thinner the further distant they are from the pipe. The layers are made of condensing and polymerizing synthetic materials. The layer thicknesses range from 20 to 50 .mu.m. However, the problem of bulging is not addressed in this reference.